In the field of electrochemistry there is a well known electrochemical cell known as a chlor-alkali cell. In this cell, an electric current is passed through a saturated brine (sodium chloride salt) solution to produce chlorine gas and caustic soda (sodium hydroxide). A large portion of the chlorine and caustic soda for the chemical and plastics industries is produced in chlor-alkali cells.
Such cells are divided by a separator into anode and cathode compartments. The separator characteristically can be a substantially hydraulically impermeable cation exchange membrane such as the commercially available NAFION manufactured by the E. I. duPont Nemours and Co. Alternatively the separator can be a porous diaphragm, e.g. asbestos which can be in the form of vacuum deposited fibers or asbestos paper sheet as are well known in the art. The anode can be a valve metal, e.g., titanium, provided with a precious metal coating to yield what is known in the art as a dimensionally stable anode. The cathodes employed in such chlor-alkali cells are generally steel. At these cathodes both caustic soda and hydrogen are produced with chlorine being produced at the anode. This is a very energy intensive process.
One of the unwanted byproducts present in such chlor-alkali cells is hydrogen which forms at the cell cathode. This hydrogen increases the power requirement for the overall electrochemical process and eliminating its formation is one of the desired results in chlor-alkali cell operation. It has been estimated that 25% of the electrical energy required to operate a chlor-alkali cell is utilized in the formation of hydrogen at the cathode. Hence, the prevention of hydrogen formation can lead to substantial energy conservation and savings in the cost of electricity required to operate the cell. In an attempt to achieve cost savings and energy conservation in operating chlor-alkali cells, attention has been directed to what are known as oxygen (air) cathodes. These cathodes prevent the formation of hydrogen at the cathode and enhance the formation of hydroxyl groups resulting in alkali which can be readily removed as product. Savings in cost for, and conservation of, electrical energy are thereby achieved.
One form of oxygen (air) cathode involves use of an active layer containing porous active carbon particles, which may or may not be catalyzed using noble metal catalyst, such as, silver, platinum, etc. The active carbon particles become flooded with the caustic soda thereby significantly reducing their ability to catalyze the formation of hydroxyl groups at the air cathode and resulting in a loss of activity of the air cathode. Some attempts to overcome this difficulty involve incorporating hydrophobic materials, e.g., polytetrafluoroethylene (PTFE) in such active layers in particulate or fibrillated (greatly attenuated and elongated) form to impart hydrophobicity to the active carbon layer, per se. Other approaches at combatting this problem utilize PTFE in a protective or backing sheet which can be laminated or otherwise attached to the active layer. Still other proposed solutions involve use of PTFE in both the active layer and in a backing sheet secured thereto. Such composite active layers, however, are often subjected to loss of strength resulting in failure combined with blistering of the electrode particularly in the active layer when the chlor-alkali cell is operated at high current densities, viz., current densities of from about 250 milliamperes/cm.sup.2 and higher.
U.S. Pat. No. 3,838,064 to John W. Vogt et al is directed to a process for dust control involving mixing a finely divided fibrillatable polytetrafluoroethylene with a material which characteristically forms a dust to form a dry mixture followed by sufficient working to essentially avoid dusting. Very small concentrations of PTFE, e.g., from about 0.02 to about 3% by weight are applied to achieve the dust control. Corresponding U.S. Pat. No. 3,838,092 also to Vogt et al is directed to dustless compositions containing fiberous polytetrafluoroethylene in concentrations of about 0.02% to less than 1%, e.g., about 0.75% by weight of PTFE based on total solids.
An article entitled "ON THE EFFECT ON VARIOUS ACTIVE CARBON CATALYSTS ON THE BEHAVIOR OF CARBON GAS-DIFUSION AIR ELECTRODES: 1. ALKALINE SOLUTIONS" by I. Iliev et al appearing in the Journal Of Power Sources, 1 (1976/1977) 35,46 Elsevier Sequoia S.A, Lausanne-printed in The Netherlands, at pages 35 to 46 of said Journal is directed to double-layer, fixed-zone polytetrafluoroethylene-bonded carbon electrodes having a gas supplying layer of carbon black "XC" wetproofed with 35% Teflon and an active layer consisting of a 30 mg/cm.sup.2 mixture of the same wetproofed material "XC-35" and active carbon (weight ratio of 1:2.5). These electrodes were sintered at 350.degree. C. under pressure of 200 kg/cm.sup.2 and employed as oxygen(air) cathodes in alkaline test environments.
The present invention is readily distinguishable from the oxygen (air) cathodes described by Iliev et al in that according to this invention, larger active carbon particles are polytetrafluoroethyleneated (discontinuously coated with much smaller PTFE particles) with subsequent fibrillation followed by heat treating and forming into a coherent, self-sustaining sheet without sintering. The active layers of this invention, when incorporated into an electrode, result in an active layer having a desirable combination of tensile strength with resistance to blistering under high current densities in use. It will be observed the conditions employed in formation of the active layer of this invention, are insufficient to effect sintering of the PTFE contained therein.
British Pat. No. 1,284,054 to Boden et al is directed to forming an air-breathing electrode containing an electrolyte within an air-depolarized cell. This air-breathing electrode is made by hot pressing a fluoropolymer sheet, containing a pore-forming agent, on to a catalyst composition (containing silver) and metallic grid member. According to page 3 of said British patent, the PTFE-pore-forming agent-paraffin wax containing wetproofing sheet is subjected to a solvent wash to remove the paraffin wax (lubricant and binder) and then sintered in a sintering furnace at the appropriate temperatures for sintering and while it still contains the pore-forming particles. It is then ready for application to the catalyst composition of the air electrode for the hot pressing operation. Hot pressing involves the use of pressures ranging from about 5,000 about 30,000 psi in conjunction with temperatures ranging from 200.degree. F. to 400.degree. F. The process of the present invention is readily distinguishable from said Boden et al British Pat. No. 1,284,054 in that the present invention avoids the use of wax, avoids the trouble and expense of removing wax and does not employ sintering and high pressures. Moreover Boden et al do not use a discrete active layer containing catalyst in their process.
The publication "Advances in Chemistry Series," copyright 1969, Robert F. Gould (Editor), American Chemical Society Publications, contains at pages 13 to 23 an article entitled "A Novel Air Electrode" by H. P. Landi et al. The electrode described contains 2 to 8 percent PTFE, is produced without sintering and is composed of graphitic carbon (ACCO Graphite) or metallized graphite carbon particles blended with a PTFE latex and a thermoplastic molding compound to form an interconnected network which enmeshes the filler particles. This blend is molded into a flat sheet and the thermoplastic is then extracted. The present process employs non-graphitic active carbons, significantly higher concentrations of PTFE in the active layer while avoiding the use of thermoplastic molding compound and avoiding the necessity to remove same. Also, the active layer of this invention is formed by rolling a prefibrillated granular mix and no molding step is necessary. No indication is given by Landi et al as to the stability and durability of their air electrode and no life testing or data is included in said article.